Methods for depleting rna from nucleic acid samples

ABSTRACT

The invention relates to methods of depleting RNA from a nucleic acid sample. The RNA may be any RNA, including, but not limited to, rRNA, tRNA, and mRNA. The method is useful for depleting RNA from a nucleic acid sample obtained from a fixed paraffin-embedded tissue (FPET) sample. The method may also be used to prepare cDNA, in particular, a cDNA library for further analysis or manipulation.

This application is a divisional of U.S. application Ser. No. 14/644,455, filed on Mar. 11, 2015, which is a divisional of U.S. application Ser. No. 12/940,981, filed Nov. 5, 2010, issued on Apr. 14, 2015, as U.S. Pat. No. 9,005,891 B2, which claims priority benefit of U.S. Provisional Application No. 61/259,934, filed on Nov. 10, 2009, the entire disclosures of which are incorporated herein by reference in their entirety.

This application contains a Sequence Listing in an ASCII text file named “GHI041USSeqListing.txt” created Nov. 5, 2010 (24,000 bytes), which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to methods of depleting undesired RNA from nucleic acid samples. The invention is useful for preparing cDNA from the RNA-depleted nucleic acid samples, for example, from fixed paraffin embedded tissue (FPET) samples.

INTRODUCTION

The most commonly used methods known in the art for the detection and/or quantification of mRNA expression in a sample include northern blotting and in situ hybridization (Parker & Barnes, Methods in Molecular Biology 106:247-283 (1999)); RNAse protection assays (Hod, Biotechniques 13:852-854 (1992)); microarrays (Schena et al., Proc. Natl. Acad. Sci. USA 93(2):106-149 (1996)), and reverse transcription polymerase chain reaction (RT-PCR) (Weis et al., Trends in Genetics 8:263-264 (1992)). Alternatively, antibodies may be employed that can recognize specific hybrids, including DNA hybrids, RNA hybrids, and DNA-RNA hybrid hybrids or DNA-protein hybrids. Of these, due to its sensitivity, reproducibility, and large dynamic range, real-time reverse transcription PCR (RT-PCR) is increasingly becoming the method of choice for high-throughput, accurate expression profiling of a large number of genes.

As RNA cannot serve as a template for PCR, the first step in gene expression profiling by RT-PCR is the reverse transcription of the RNA template into cDNA, followed by its exponential amplification in a PCR reaction. The conversion of mRNA to cDNA is typically performed by oligo dT priming, random priming, or gene-specific priming of the mRNA in the presence of the reverse transcriptase (RT) enzyme. However, some samples may contain additional material that may interfere with the detection and/or quantification of the mRNA. For example, fixed, paraffin-embedded tissue samples may be archived for as long as 10-20 years and may be badly degraded. RNA isolated from such samples may be difficult to efficiently convert into cDNA.

SUMMARY

It is often desirable to isolate, enrich, or increase sequences within a much larger population of sequences in order to limit analysis to those sequences of interest and to reduce interference and unnecessary work that may be caused by the presence of undesirable sequences. The disclosed invention provides a novel method that depletes undesired sequences from a sample containing a mixture of sequences and therefore, enriches for a population of interest.

The present invention provides a method of depleting ribonucleic acid (RNA) from a nucleic acid sample. The method comprises contacting the sample with at least one deoxyribonucleic (DNA) probe complementary to a target RNA. In an embodiment of the invention, the sample is contacted with a single DNA probe. In a further embodiment, the sample is contacted with a multiplicity of DNA probes. In another embodiment, the DNA probe(s) specifically hybridizes to substantially the entire full-length sequence or the entire full-length sequence of the target RNA and forms a DNA-RNA hybrid. The sample is then contacted with a nuclease enzyme, for example a a ribonuclease, and the nuclease enzyme degrades the target RNA in the DNA-RNA hybrid. In an embodiment of the invention, the nuclease enzyme is RNase H, for example a thermostable RNase H. Optionally, the sample may be additionally contacted with a deoxyribonuclease (DNase) to degrade the DNA probe. In one embodiment, the DNase is DNase I. In another embodiment of the invention, the target RNA comprises less than 10% of all RNA present in the nucleic acid sample after RNA depletion.

In a further embodiment of the invention, the target RNA is ribosomal RNA (rRNA), transfer RNA (tRNA) and/or messenger RNA (mRNA). The target rRNA may be eukaryotic rRNA. In a specific embodiment of the invention, the target rRNA is the cytoplasmic 28S or 18S rRNA, or both. In another embodiment, the target rRNA is the mitochondrial rRNA 12S or 16S rRNA, or both.

In an embodiment of the invention, the target RNA is mRNA. The target mRNA may include cellular transcripts that are highly expressed. Thus, in an embodiment of the invention, the target mRNA is selected from GAPDH, ACTB, and TUBB.

In yet another embodiment of the invention, the nucleic acid sample comprises total RNA or amplified RNA. In a specific embodiment, the nucleic acid sample is obtained from a fixed paraffin-embedded tissue (FPET) sample. Thus, the method of the invention can be used to deplete RNA from a nucleic acid sample obtained from an FPET sample. The FPET sample may be from a tumor, and the tumor may be a cancer. In an embodiment of the invention, the cancer is selected from breast cancer, colon cancer, lung cancer, prostate cancer, hepatocellular cancer, gastric cancer, pancreatic cancer, cervical cancer, ovarian cancer, liver cancer, bladder cancer, cancer of the urinary tract, thyroid cancer, renal cancer, carcinoma, melanoma, and brain cancer.

The method of the present invention can be further used to prepare cDNA useful for further analysis. In this aspect, cDNA is prepared from the nucleic acid sample after RNA depletion. The cDNA may be purified and/or amplified and may be a cDNA library. The cDNA may be used for further analysis, such as sequencing and/or gene expression analysis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a method for depleting RNA from a nucleic acid sample comprising total or amplified RNA. When the starting material is amplified RNA and the RNA is amplified as illustrated in FIG. 2 and described below, the resulting RNA comprises a T7 promoter sequence (T7) appended at the end of each strand.

FIG. 2 is a schematic illustration of a method for amplifying RNA.

FIG. 3 is a graph showing the relative abundance of 18S and 28S rRNA in mock treated (blank boxes), RNase H-treated (filled boxes), and RNase H and DNase I-treated (hatched boxes) samples. RT-PCR was performed on each sample in triplicate for three different regions of 18S and 28S rRNAs. β-actin and RPL were used as controls.

FIG. 4 shows the average difference correlation between amplified RNA before rRNA-depletion (“amplified RNA”) and after rRNA-depletion (“depleted method1 RNA”).

FIG. 5 shows the average difference correlation between amplified RNA before rRNA-depletion (“amplified RNA”) and after rRNA-depletion and DNase I treatment (“depleted method2 RNA”).

FIG. 6 is a schematic illustration of a method for preparing a cDNA library from amplified RNA.

FIG. 7 shows the average difference correlation of total unamplified RNA (“source”) versus amplified Lo ER RNA (“amplified”).

FIG. 8 shows the average difference correlation of total unamplified RNA (“source”) versus amplified Hi ER RNA (“amplified”).

FIG. 9 is a graph showing the relative abundance of 18S and 28S rRNA in untreated (blank boxes), RNase H-treated (filled boxes), and RNase H and DNase I-treated (hatched boxes) samples. RT-PCR was performed on each sample in triplicate for three different regions of 18S and 28S rRNAs. β-actin and RPL were used as controls.

FIG. 10 shows the average difference correlation of rRNA-undepleted Lo ER amplified RNA and rRNA-depleted Lo ER amplified RNA treated with DNase I.

FIG. 11 shows the average difference correlation of rRNA-undepleted Hi ER amplified RNA and rRNA-depleted Hi ER amplified RNA treated with DNase I.

FIG. 12 shows the average difference correlation of duplicates of rRNA-depleted Hi ER amplified RNA.

FIG. 13 shows a comparison of the size distribution between the cDNA libraries prepared by the method of the invention and that of a cDNA library obtainable from Illumina, Inc. (San Diego, Calif.).

FIG. 14 is a schematic representation of the cDNA inserts generated in the cDNA library. Symbols: P1 and P2 are adapter sequences (SEQ ID NOs: 89 and 90, respectively); barcode is a 6 base pair sequence that is unique to a sample; T7 is a 16 base pair sequence containing the T7 promoter sequence (SEQ ID NO: 91); poly A is a poly A tract complementary to a poly T tract.

DETAILED DESCRIPTION

Unless defined otherwise, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. For example, Singleton et al., Dictionary of Microbiology and Molecular Biology 2nd ed., J. Wiley & Sons (New York, N.Y. 1994), provide one skilled in the art with a general guide to many of the terms used in the present application.

One skilled in the art will recognize many methods and materials similar or equivalent to those described herein, which could be used in the practice of the present invention. Indeed, the present invention is in no way limited to the methods and materials described. For purposes of the present invention, the following terms are defined below.

Additionally, the practice of the present invention will employ, unless otherwise indicated, conventional techniques of molecular biology (including recombinant techniques), microbiology, cell biology, and biochemistry, which are within the skill of the art. Such techniques are explained fully in the literature, such as, “Molecular Cloning: A Laboratory Manual”, 2^(nd) edition (Sambrook et al., 1989); “Oligonucleotide Synthesis” (M. J. Gait, ed., 1984); “Animal Cell Culture” (R. I. Freshney, ed., 1987); “Methods in Enzymology” (Academic Press, Inc.); “Handbook of Experimental Immunology”, 4^(th) edition (D. M. Weir & C. C. Blackwell, eds., Blackwell Science Inc., 1987); “Gene Transfer Vectors for Mammalian Cells” (J. M. Miller & M. P. Calos, eds., 1987); “Current Protocols in Molecular Biology” (F. M. Ausubel et al., eds., 1987); and “PCR: The Polymerase Chain Reaction”, (Mullis et al., eds., 1994).

As used herein, the term “nucleic acid” refers to a deoxyribonucleotide or ribonucleotide polymer. The nucleic acid may be unmodified RNA or DNA or modified RNA or DNA and includes, without limitation, single- and double-stranded DNA, DNA including single- and double-stranded regions, single- and double-stranded RNA, and RNA including single- and double-stranded regions, hybrid molecules comprising DNA and RNA that may be single-stranded or, more typically, double-stranded or include single- and double-stranded regions. In general, the term “nucleic acid” embraces all chemically, enzymatically and/or metabolically modified forms of unmodified nucleic acids, as well as the chemical forms of DNA and RNA characteristic of viruses and cells, including simple and complex cells.

As used herein, the term “nucleic acid sample” refers to a sample containing both desired and undesired nucleic acids. As a non-limiting example, a nucleic acid sample comprises total genomic DNA, total cellular RNA, or a combination thereof. Moreover, a nucleic acid sample may have been enriched for a given population of nucleic acids but includes other undesirable nucleic acids. For example, a nucleic acid sample may be a sample that has been enriched for desired messenger RNA (mRNA) but still includes some undesired ribosomal RNA (rRNA). General methods for mRNA extraction are well known in the art and are disclosed in standard textbooks of molecular biology, including Ausubel et al., Current Protocols of Molecular Biology, John Wiley and Sons (1997). Methods for RNA extraction from paraffin embedded tissues are disclosed, for example, in Rupp and Locker, Lab Invest. 56:A67 (1987), and De Andrés et al., BioTechniques 18:42044 (1995), and in U.S. Pat. No. 7,081,340, issued Jul. 25, 2006, all of which are herein incorporated by reference. In particular, RNA isolation can be performed using a purification kit, buffer set and protease from commercial manufacturers, such as Qiagen, according to the manufacturer's instructions. For example, total RNA from cells in culture can be isolated using Qiagen RNeasy mini-columns. Other commercially available RNA isolation kits include MasterPure™ Complete DNA and RNA Purification Kit (EPICENTRE®, Madison, Wis.), and Paraffin Block RNA Isolation Kit (Ambion, Inc.). Total RNA from tissue samples can be isolated using RNA Stat-60 (Tel-Test). RNA prepared from tumor can be isolated, for example, by cesium chloride density gradient centrifugation.

If necessary, DNA can be removed at various stages of RNA isolation, by DNase or other techniques well known in the art. After analysis of the RNA concentration after purification, RNA repair and/or amplification steps may be performed before subjecting the RNA to further manipulation or analysis. For example, a nucleic acid sample may be a sample in which the mRNA has been amplified by, for example, in vitro transcription. Exemplary methods for RNA amplification are disclosed in U.S. Pat. No. 7,081,340, issued Jul. 25, 2006; U.S. Pub. No. 2005/0196782, published Sep. 8, 2005; and U.S. Pub. No. 2009/0042192, published Feb. 12, 2009, all of which are hereby incorporated by reference. Other methods for RNA amplification known in the art may also be used.

Furthermore, in an embodiment of the invention, the nucleic acid sample is obtained from cells, tissues, organs, or lysates, fractionations, or portions thereof, from any human or non-human animal, and may be fresh, fixed, or frozen. In a particular embodiment, the nucleic acid sample is obtained from a biopsy sample from a human and may be derived from a fine needle, core, or any other types of biopsy. In yet another particular embodiment, the nucleic acid sample is obtained from a fixed paraffin-embedded tissue (FPET) sample from a human.

As used herein, the term “DNA probe” refers to a single-stranded DNA oligonucleotide having a sequence partly or completely complementary to a “target RNA” and specifically hybridizes to the RNA. As used herein, “target RNA” refers to an undesired RNA that is the target for depletion from the nucleic acid sample. The target RNA may be any RNA, including, but not limited to, rRNA, tRNA, and mRNA. DNA probes may be produced by techniques known in the art such as chemical synthesis and by in vitro or in vivo expression from recombinant nucleic acid molecules. The DNA probes may also be produced by amplification of the target RNA, including, but not limited to, RT-PCR, asymmetric PCR, and rolling circle amplification. In an embodiment of the invention, DNA probes are 10 to 100 nucleotides in length. In another embodiment, the DNA probes are 14 to 85 nucleotides in length. In yet another embodiment, the DNA probes are 50 to 80 nucleotides in length. In a further embodiment of the invention, the DNA probes are 60 to 80 nucleotides in length. In yet a further embodiment, a single DNA probe spans the entire length of the target RNA. DNA probes may or may not have regions that are not complementary to a target RNA, so long as such sequences do not substantially affect specific hybridization to the RNA. In another embodiment of the invention, the DNA probe may be complementary to all or part of a target RNA sequence and therefore, there may be more than one DNA probe that specifically hybridizes to the RNA. For example, there may be at least 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 DNA probes that specifically hybridize to an RNA. In a particular embodiment, at least one DNA probe specifically hybridizes to substantially the entire full length sequence of the target RNA. As used herein, “substantially the entire full length sequence” refers to less than 100% but at least 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%, or any range therein, of the full length sequence. In an embodiment of the invention, a multiplicity of DNA probes specifically binds to substantially the entire full length sequence of the target RNA. In yet another embodiment, a multiplicity of DNA probes specifically binds to the entire full length sequence of the target RNA. The DNA probes may be complementary to sequences that overlap one another, or may be complementary to non-overlapping sequences.

As used herein, “specifically hybridizes” refers to a state where a specific DNA probe is able to hybridize with a target RNA, for example, rRNA, over other nucleic acids present in a nucleic acid sample. The DNA probe is first denatured into single-stranded DNA by methods known in the art, for example, by heating or under alkaline conditions, and then hybridized to the target RNA by methods also known in the art, for example, by cooling the heated DNA in the presence of the target RNA. The condition under which a DNA probe specifically hybridizes with an RNA are well known to those of ordinary skill in the art and it will be appreciated that these conditions may vary depending upon factors including the GC content and length of the probe, the hybridization temperature, the composition of the hybridization reagent or solution, and the degree of hybridization specificity sought.

In an embodiment of the invention, the DNA probe is specifically hybridized at a temperature range of between 22° C. and 100° C. In another embodiment of the invention, the DNA probe is specifically hybridized at a temperature range of between 50° C. and 98° C. In yet another embodiment, the DNA probe is specifically hybridized at about 95° C. The hybridization temperature may also be ramped down to stabilize the hybridization. For example, the hybridization temperature may be ramped down at a rate of 0.1° C./sec to 22° C. after hybridization at 95° C.

As used herein, the term “complementary” refers to a nucleic acid comprising a sequence of consecutive nucleobases capable of hybridizing to another nucleic acid strand even if less than all the nucleobases do not base pair with a counterpart nucleobase. In certain embodiments, a “complementary” nucleic acid comprises a sequence in which at least 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%, and any range therein, of the nucleobase sequence is capable of base-pairing with another nucleic acid sequence.

In an embodiment of the invention, a DNA probe may be labeled with a reporter group moiety such as a radioisotope, a fluorescent or chemiluminescent moiety, with an enzyme or ligand, which can be used for detection or confirmation that the probe has hybridized to the target sequence.

In embodiments in which the target RNA is rRNA, the rRNA may be eukaryotic rRNA, and may be either cytoplasmic rRNA or mitochondrial rRNA. Cytoplasmic rRNAs include, for example, 28S, 5.8S, 5S and 18S rRNAs. Mitochondrial rRNAs include, for example, 12S and 16S rRNAs. The rRNA may also be prokaryotic rRNA, which includes, for example, 5S, 16S, and 23S rRNA. The sequences for rRNAs are well known to those skilled in the art and can be readily found in sequence databases such as GenBank or may be found in the literature. For example, the sequence for the human 18S rRNA can be found in GenBank as Accession No. M10098 and the human 28S rRNA as Accession No. M11167.

In another embodiment, the target RNA is transfer RNA (tRNA). In yet another embodiment, the target RNA may be a particular mRNA. For example, it may be desirable to remove cellular transcripts that are usually present in abundance. Thus, the target mRNA includes, but is not limited to, ACTB, GAPDH, and TUBB. Other sequences for tRNA and specific mRNA are well known to those skilled in the art and can be readily found in sequence databases such as GenBank or may be found in the literature.

In a particular embodiment of the invention, mRNA is not targeted for depletion and therefore, the DNA probe does not have a poly-T that will hybridize to the poly-A tail of eukaryotic mRNA. In yet another particular embodiment of the invention, the DNA probe targets and specifically hybridizes to human 18S or human 28S rRNA. Examples of the sequences of the DNA probes targeting the full length sequences of human 18S and human 28S rRNA are shown in Table 1.

The term “tumor,” as used herein, refers to all neoplastic cell growth and proliferation, whether malignant or benign, and all pre-cancerous and cancerous cells and tissues.

The terms “cancer” and “cancerous” refer to or describe the physiological condition in mammals that is typically characterized by unregulated cell growth. Examples of cancer include but are not limited to, breast cancer, colon cancer, lung cancer, prostate cancer, hepatocellular cancer, gastric cancer, pancreatic cancer, cervical cancer, ovarian cancer, liver cancer, bladder cancer, cancer of the urinary tract, thyroid cancer, renal cancer, carcinoma, melanoma, and brain cancer.

TABLE 1 SEQ Sequence ID Name Sequence (5′-3′) Length NO. T5319/ TTAATCATGGCCTCAGTTCCGAAAACCAACAAAATAGAACCGCGGTCCTATTCCATTATTCCTAGCTGCGGTATCCAGGC 80  1 18srRNA.R6 T5320/ GCTTTCGCTCTGGTCCGTCTTGCGCCGGTCCAAGAATTTCACCTCTAGCGGCGCAATACGAATGCCCCCGGCCGTCCCTC 80  2 18srRNA.R6 T5321/ TGGTCGGAACTACGACGGTATCTGATCGTCTTCGAACCTCCGACTTTCGTTCTTGATTAATGAAAACATTCTTGGCAAAT 80  3 18srRNA.R6 T5322/ TCCTCGTTCATGGGGAATAATTGCAATCCCCGATCCCCATCACGAATGGGGTTCAACGGGTTACCCGCGCCTGCCGGCGT 80  4 18srRNA.R6 T5323/ ATCGGTAGTAGCGACGGGCGGTGTGTACAAAGGGCAGGGACTTAATCAACGCAAGCTTATGACCCGCACTTACTGGGAAT 80  5 18srRNA.R6 T5324/ TTCGACCGTCTTCTCAGCGCTCCGCCAGGCCGTGGGCCGACCCCGGCGGGGCCGATCCGAGGGCCTCACTAAACCATCCA 80  6 18srRNA.R6 T5325/ GGGGGCTCGAGGACGGGCCCGGCGCCCCGCAAGCGAGGAGGACGACGGACGGACGG 56  7 18srRNA.R6 T5326/ TTTGAGACAAGCATATGCTACTGGCAGGATCAACCAGGTAGGTAGGTAGAGCGCGGCGAGGCCCCGACGCGGCCGGACGG 80  8 18srRNA.R6 T5327/ AAGGAACCATAACTGATTTAATGAGCCATTCGCAGTTTCACTGTACCGGCCGTGCGTACTTAGACATGCATGGCTTAATC 80  9 18srRNA.R6 T5328/ CGAAGGGGGTCAGCGCCCGTCGGCATGTATTAGCTCTAGAATTACCACAGTTATCCAAGTAGGAGAGGAGCGAGCGACCA 80 10 18srRNA.R6 T5329/ CCGCGGCCCGCCCCCCGGCCGGGGCCGGAGAGGGGCTGACCGGGTTGGTTTTGATCTGATAAATGCACGCATCCCCCCCG 80 11 18srRNA.R6 T5330/ TAGGGCAGACGTTCGAATGGGTCGTCGCCGCCACGGGGGGCGTGCGATCGGCCCGAGGTTATCTAGAGTCACCAAAGCCG 80 12 18srRNA.R6 T5331/ CTCCCTCTCCGGAATCGAACCCTGATTCCCCGTCACCCGTGGTCACCATGGTAGGCACGGCGACTACCATCGAAAGTTGA 80 13 18srRNA.R6 T5332/ TTCGTCACTACCTCCCCGGGTCGGGAGTGGGTAATTTGCGCGCCTGCTGCCTTCCTTGGATGTGGTAGCCGTTTCTCAGG 80 14 18srRNA.R6 T5333/ CTCCAATGGATCCTCGTTAAAGGATTTAAAGTGGACTCATTCCAATTACAGGGCCTCGAAAGAGTCCTGTATTGTTATTT 80 15 18srRNA.R6 T5334/ CTACGAGCTTTTTAACTGCAGCAACTTTAATATACGCTATTGGAGCTGGAATTACCGCGGCTGCTGGCACCAGACTTGCC 80 16 18srRNA.R6 T5335/ AGGGGGCGCCGAGAGGCAAGGGGCGGGGACGGGCGGTGGCTCGCCTCGCGGCGGACCGCCCGCCCGCTCCCAAGATCCAA 80 17 18srRNA.R6 T5336/ GGCTCGGGCCTGCTTTGAACACTCTAATTTTTTCAAAGTAAACGCTTCGGGCCCCGCGGGACACTCAGCTAAGAGCATCG 80 18 18srRNA.R6 T5337/ ACCCAAAGACTTTGGTTTCCCGGAAGCTGCCCGGCGGGTCATGGGAATAACGCCGCCGCATCGCCGGTCGGCATCGTTTA 80 19 18srRNA.R6 T5338/ TAAGCCGCAGGCTCCACTCCTGGTGGTGCCCTTCCGTCAATTCCTTTAAGTTTCAGCTTTGCAACCATACTCCCCCCGGA 80 20 18srRNA.R6 T5339/ CACGGAATCGAGAAAGAGCTATCAATCTGTCAATCCTGTCCGTGTCCGGGCCGGGTGAGGTTTCCCGTGTTGAGTCAAAT 80 21 18srRNA.R6 T5340/ AGCATGCCAGAGTCTCGTTCGTTATCGGAATTAACCAGACAAATCGCTCCACCAACTAAGAACGGCCATGCACCACCACC 80 22 18srRNA.R6 T5341/ TCAATCTCGGGTGGCTGAACGCCACTTGTCCCTCTAAGAAGTTGGGGGACGCCGACCGCTCGGGGGTCGCGTAACTAGTT 80 23 18srRNA.R6 T5342/ AGGGTAGGCACACGCTGAGCCAGTCAGTGTAGCGCGCGTGCAGCCCCGGACATCTAAGGGCATCACAGACCTGTTATTGC 80 24 18srRNA.R6 T5343/ TAATGATCCTTCCGCAGGTTCACCTACGGAAACCTTGTTACGACTTTTACTTCCTCTAGATAGTCAAG 68 25 18srRNA.R6 T5344/ GGAATCCTGGTTAGTTTCTTTTCCTCCGCTGACTAATATGCTTAAATTCAGCGGGTCGCCACGTCTGATCTGAGGTCGCG 80 26 28srRNA.R3 T5345/ TTCCGTACGCCACATGTCCCGCGCCCCGCGGGGCGGGGATTCGGCGCTGGGCTCTTCCCTGTTCACTCGCCGTTACTGAG 80 27 28srRNA.R3 T5346/ TACCGGCCTCACACCGTCCACGGGCTGGGCCTCGATCAGAAGGACTTGGGCCCCCCACGAGCGGCGCCGGGGAGCGGGTC 80 28 28srRNA.R3 T5347/ TTAGATGGAGTTTACCACCCGCTTTGGGCTGCATTCCCAAGCAACCCGACTCCGGGAAGACCCGGGCGCGCGCCGGCCGC 80 29 28srRNA.R3 T5348/ CTTGAACTCTCTCTTCAAAGTTCTTTTCAACTTTCCCTTACGGTACTTGTTGACTATCGGTCTCGTGCCGGTATTTAGCC 80 30 28srRNA.R3 T5349/ GCCGGACCCGCCGCCGGGTTGAATCCTCCGGGCGGACTGCGCGGACCCCACCCGTTTACCTCTTAACGGTTTCACGCCCT 80 31 28srRNA.R3 T5350/ GGGGCGGCGGGGGAAGGGAGGGCGGGTGGAGGGGTCGGGAGGAACGGGGGGCGGGAAAGATCCGCCGGGCCGCCGACACG 80 32 28srRNA.R3 T5351/ TCCCCCGCCGACCCCACCCCCGGCCCCGCCCGCCCACCCCCGCACCCGCCGGAGCCCGCCCCCTCCGGGGAGGAGGAGGA 80 33 28srRNA.R3 T5352/ TCCCAGCCGTCCCGGAGCCGGTCGCGGCGCACCGCCTGGAAATGCGCCCGGCGGCGGCCGGTCGCCGGTCCGGGGGACGG 80 34 28srRNA.R3 T5353/ GGGGCCGGGGGGCGGAGACGGGGGAGGAGGAGGACGGACGGACGGACGGGGCCCCCCGAGCCACCTTCCCCGCCGGGCCT 80 35 28srRNA.R3 T5354/ CGGTCCCGCCGCCCCCGCCGCCGCCGCCACCGCCGCCGCCGCCGCCGCCCCGACCCGCGCGCCCTCCCGAGGGAGGACGC 80 36 28srRNA.R3 T5355/ CGGCGACGGGTCTCGCTCCCTCGGCCCCGGGATTCGGCGAGTGCTGCTGCCGGGGGGGCTGTAACACTCGGGGGGGGTTT 80 37 28srRNA.R3 T5356/ AGGAGACGCCGGCGCCGCGCCGGGGGAGACCCCCCTCGCGGGGATTCCCGCGGGGGTGGGCGCCGGGAGGGGGGAGAGCG 80 38 28srRNA.R3 T5357/ CCCGTCGCCGGGGCGGGGGCGCGGGGAGGAGGGGTGGGAGAGCGGTCGCGCCGTGGGAGGGGTGGCCCGGCCCCCCCACG 80 39 28srRNA.R3 T5358/ CCCCGGGCCCGACGGCGCGACCCGCCCGGGGCGCACTGGGGACAGTCCGCCCCGCCCCCCGACCCGCGCGCGGCACCCCC 80 40 28srRNA.R3 T5359/ CCGACGTCGCCGCCGACCCCGTGCGCTCGCTCCGCCGTCCCCCTCTTCGGGGGACGCGCGCGTGGCCCCGAGAGAACCTC 80 41 28srRNA.R3 T5360/ CGGCGGCTTTCGTGCGAGCCCCCGACTCGCGCACGTGTTAGACTCCTTGGTCCGTGTTTCAAGACGGGTCGGGTGGGTAG 80 42 28srRNA.R3 T5361/ GGTGCCCCTCGGCGGACTGGAGAGGCCTCGGGATCCCACCTCGGCCGGCGAGCGCGCCGGCCTTCACCTTCATTGCGCCA 80 43 28srRNA.R3 T5362/ GGCATAGTTCACCATCTTTCGGGTCCTAACACGTGCGCTCGTGCTCCACCTCCCCGGCGCGGCGGGCGAGACGGGCCGGT 80 44 28srRNA.R3 T5363/ TACCCAGGTCGGACGACCGATTTGCACGTCAGGACCGCTACGGACCTCCACCAGAGTTTCCTCTGGCTTCGCCCTGCCCA 80 45 28srRNA.R3 T5364/ TCTGCGAGAGCGCCAGCTATCCTGAGGGAAACTTCGGAGGGAACCAGCTACTAGATGGTTCGATTAGTCTTTCGCCCCTA 80 46 28srRNA.R3 T5365/ ATAGGTTGAGATCGTTTCGGCCCCAAGACCTCTAATCATTCGCTTTACCGGATAAAACTGCGTGGCGGGGGTGCGTCGGG 80 47 28srRNA.R3 T5366/ AAAGTGGCCCACTAGGCACTCGCATTCCACCCCGGCTCCACGCCAGCGAGCCGGGCTTCTTACCCATTTAAAGTTTGAGA 80 48 28srRNA.R3 T5367/ TTCTGGGGTCTGATGAGCGTCGGCATCGGGCGCCTTAACCCGGCGTTCGGTTCATCCCGCAGCGCCAGTTCTGCTTACCA 80 49 28srRNA.R3 T5368/ GCAGGTGAGTTGTTACACACTCCTTAGCGGATTCCGACTTCCATGGCCACCGTCCTGCTGTCTATATCAACCAACACCTT 80 50 28srRNA.R3 T5369/ CGTCCACTCTCGACTGCCGGCGACGGCCGGGTATGGGCCCGACGCTCCAGCGCCATCCATTTTCAGGGCTAGTTGATTCG 80 51 28srRNA.R3 T5370/ CCCACACCCCCGCCGCCGCCGCCGCCGCCGCCCTCCGACGCACACCACACGCGCGCGCGCGCCGCCGCCCCCGCCGCTCC 80 52 28srRNA.R3 T5371/ TCCGCGGGGCTCCGGGGGCGGGGAGCGGGGCGTGGGCGGGAGGAGGGGAGGAGGCGTGGGGGGGGGGGCGGGGGAAGGAC 80 53 28srRNA.R3 T5372/ CCTGCGGCGGCCTCCACCCGGGCCCGCGCCCTAGGCTTCAAGGCTCACCGCAGCGGCCCTCCTACTCGTCGCGGCGTAGC 80 54 28srRNA.R3 T5373/ TGCTGTTCACATGGAACCCTTCTCCACTTCGGCCTTCAAAGTTCTCGTTTGAATATTTGCTACTACCACCAAGATCTGCA 80 55 28srRNA.R3 T5374/ CCGAGGGCAACGGAGGCCATCGCCCGTCCCTTCGGAACGGCGCTCGCCCATCTCTCAGGACCGACTGACCCATGTTCAAC 80 56 28srRNA.R3 T5375/ GTTACCGCACTGGACGCCTCGCGGCGCCCATCTCCGCCACTCCGGATTCGGGGATCTGAACCCGACTCCCTTTCGATCGG 80 57 28srRNA.R3 T5376/ CCATTCCAGGGCGCCCTGCCCTTCACAAAGAAAAGAGAACTCTCCCCGGGGCTCCCGCCGGCTTCTCCGGGATCGGTCGC 80 58 28srRNA.R3 T5377/ TCAAGGGCCAGCGAGAGCTCACCGGACGCCGCCGGAACCGCGACGCTTTCCAAGGCACGGGCCCCTCTCTCGGGGCGAAC 80 59 28srRNA.R3 T5378/ CCAGAGGCTGTTCACCTTGGAGACCTGCTGCGGATATGGGTACGGCCCGGCGCGAGATTTACACCCTCTCCCCCGGATTT 80 60 28srRNA.R3 T5379/ CCCAGCCCTTAGAGCCAATCCTTATCCCGAAGTTACGGATCCGGCTTGCCGACTTCCCTTACCTACATTGTTCCAACATG 80 61 28srRNA.R3 T5380/ TGCCCCGGGCGTGGGGGGGGCGCGCGCCTCGTCCAGCCGCGGCGCGCGCCCAGCCCCGCTTCGCGCCCCAGCCCGACCGA 80 62 28srRNA.R3 T5381/ AGAGAGAGAGAGAGAGGGCGCGGGGTGGGGAGGGAGCGAGCGGCGCGCGCGGGTGGGGCGGGGGAGGGCCGCGAGGGGGG 80 63 28srRNA.R3 T5382/ CCTGCCGCCCCGACCCTTCTCCCCCCGCCGCGCCCCCACGCGGCGCTCCCCCGGGGAGGGGGGAGGACGGGGAGCGGGGG 80 64 28srRNA.R3 T5383/ GCCGCCGGCCCCCCGGGTCCCCGGGGCCCCCCTCGCGGGGACCTGCCCCCGCCGGCCGCCCCGGCGGCCGCCGCGCGGCC 80 65 28srRNA.R3 T5384/ CTCCCCGGGGGCGGCCGCGACGCCCGCCGCAGCTGGGGCGATCCACGGGAAGGGCCCGGCTCGCGTCCAGAGTCCGCGCC 80 66 28srRNA.R3 T5385/ CGACCGCTCCCGCCCCCAGCGGACGCGCGCGCGACCGAGACGTGGGGTGGGGGTGGGGGGCGCGCCGCGCCGCCGCCGGG 80 67 28srRNA.R3 T5386/ GAACGGGGGGCGGACGGGGCCGGGGGGGTAGGGCGGGGGGACGAACCGCCCCGCCCCGCCGCCCGCCGACCGCCGCCGCC 80 68 28srRNA.R3 T5387/ GGCGGACCCGGCGGGGGGGACCGGCCCGCGGCCCCTCCGCCGCCTGCCGCCGCCGCCGCCGCGCGCCGAGGAGGAGGGGG 80 69 28srRNA.R3 T5388/ ATTCCCCTGGTCCGCACCAGTTCTAAGTCGGCTGCTAGGCGCCGGCCGAGGCGAGGCGCGCGCGGAACCGCGGCCCCGGG 80 70 28srRNA.R3 T5389/ TCAGAGCACTGGGCAGAAATCACATCGCGTCAACACCCGCCGCGGGCCTTCGCGATGCTTTGTTTTAATTAAACAGTCGG 80 71 28srRNA.R3 T5390/ GAGGCATTTGGCTACCTTAAGAGAGTCATAGTTACTCCCGCCGTTTACCCGCGCTTCATTGAATTTCTTCACTTTGACAT 80 72 28srRNA.R3 T5391/ TGGCTGTGGTTTCGCTGGATAGTAGGTAGGGACAGTGGGAATCTCGTTCATCCATTCATGCGCGTCACTAATTAGATGAC 80 73 28srRNA.R3 T5392/ CATGTCTCTTCACCGTGCCAGACTAGAGTCAAGCTCAACAGGGTCTTCTTTCCCCGCTGATTCCGCCAAGCCCGTTCCCT 80 74 28srRNA.R3 T5393/ CAGGGCCGCGGACCCCGCCCCGGGCCCCTCGCGGGGACACCGGGGGGGCGCCGGGGGCCTCCCACTTATTCTACACCTCT 80 75 28srRNA.R3 T5394/ AGAGCCCCTCGGGCTCGCCCCCCCGCCTCACCGGGTCAGTGAAAAAACGATCAGAGTAGTGGTATTTCACCGGCGGCCCG 80 76 28srRNA.R3 T5395/ CGCCCCAGTCAAACTCCCCACCTGGCACTGTCCCCGGAGCGGGTCGCGCCCGGCCGGGCGGGCGCTTGGCGCCAGAAGCG 80 77 28srRNA.R3 T5396/ TTGCCCTTCTGCTCCACGGGAGGTTTCTGTCCTCCCTGAGCTCGCCTTAGGACACCTGCGTTACCGTTTGACAGGTGTAC 80 78 28srRNA.R3 T5397/ AAACCCAAAAGGTCAGAAGGATCGTGAGGCCCCGCTTTCACGGTCTGTATTCGTACTGAAAATCAAGATCAAGCGAGCTT 80 79 28srRNA.R3 T5398/ CAAAAAGCGACGTCGCTATGAACGCTTGGCCGCCACAAGCCAGTTATCCCTGTGGTAACTTTTCTGACACCTCCTGCTTA 80 80 28srRNA.R3 T5399/ CGTTCCCTATTAGTGGGTGAACAATCCAACGCTTGGCGAATTCTGCTTCACAATGATAGGAAGAGCCGACATCGAAGGAT 80 81 28srRNA.R3 T5400/ CTGAGCAGGATTACCATGGCAACAACACATCATCAGTAGGGTAAAACTAACCTGTCTCACGACGGTCTAAACCCAGCTCA 80 82 28srRNA.R3 T5401/ CCCACAGATGGTAGCTTCGCCCCATTGGCTCCTCAGCCAAGCACATACACCAAATGTCTGAACCTGCGGTTCCTCTCGTA 80 83 28srRNA.R3 T5402/ CCGAGGCCAACCGAGGCTCCGCGGCGCTGCCGTATCGTTCGCCTGGGCGGGATTCTGACTTAGAGGCGTTCAGTCATAAT 80 84 28srRNA.R3 T5403/ GCGGGGCACGCGCCCTCCCGCGGCGGGGCGCGTGGAGGGGGGGGCGGCCCGCCGGCGGGGACAGGCGGGGGACCGGCTAT 80 85 28srRNA.R3 T5404/ AGGGGGCGGCCGCCTTTCCGGCCGCGCCCCGTTTCCCAGGACGAAGGGCACTCCGCACCGGACCCCGGTCCCGGCGCGCG 80 86 28srRNA.R3 T5405/ CGAAACCCCGACCCAGAAGCAGGTCGTCTACGAATGGTTTAGCGCCAGGTTCCCCACGAACGTGCGGTGCGTGACGGGCG 80 87 28srRNA.R3 T5406/ GACAAACCCTTGTGTCGAGGGCTGACTTTCAATAGATCGCAGCGAGGGAGCTGCTCTGCTACGTA 65 88 28srRNA.R3

FIG. 1 illustrates an embodiment of the invention. A nucleic acid sample is contacted with at least one DNA probe complementary to a target RNA. Upon hybridization, the DNA probe forms a complex with the target RNA and forms a DNA-RNA hybrid. In one embodiment, the DNA probe specifically hybridizes with an rRNA and forms a DNA-rRNA hybrid. In another embodiment, the DNA probe specifically hybridizes with a tRNA and forms a DNA-tRNA hybrid. In yet another embodiment, the DNA specifically hybridizes with an mRNA and forms a DNA-mRNA hybrid.

The DNA-RNA hybrid is then depleted from the nucleic acid sample. For example, in some embodiments, a ribonuclease (RNAse) that specifically targets DNA-RNA hybrids is used to deplete the DNA-RNA hybrid. In a specific embodiment of the invention, RNAse H is used to specifically hydrolyze the RNA in the DNA-RNA hybrid so that the RNA becomes degraded. The remaining DNA is then available to hybridize with another RNA target sequence. In an embodiment of the invention, the RNAse H is a thermostable RNAse H. RNAse H may be obtained commercially, including, for example, Hybridase™ (EPICENTRE Biotechnologies, Madison, Wis.).

In an embodiment of the invention, the RNAse H is allowed to degrade the DNA-RNA hybrid at a temperature range of between 32° C. and 70° C. In another embodiment, the RNAse H is allowed to degrade the DNA-RNA hybrid at a temperature range of between 37° C. and 60° C. In yet another embodiment, RNAse H is allowed to degrade the DNA-RNA hybrid at a temperature of about 37° C.

The DNA probe hybridization step and the RNA degradation step with RNAse H may be repeated.

If desired, the DNA may also be removed by addition of an enzyme that specifically targets and digests DNA. In an embodiment of the invention, a DNAse is used. In a specific embodiment of the invention, the DNAse is DNAse I. DNAses may be obtained commercially.

Upon depletion of the undesired nucleic acid, the sample will contain an enriched population of the desired nucleic acid. In an embodiment of the invention, the undesired nucleic acid comprises less than 20%, 19%, 18%, 17%, 16% 15%, 14%, 13%, 12%, 11% 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%, or any range therein, of the total nucleic acid in the sample after depletion of the undesired nucleic acid. Thus, the enriched population of the desired nucleic acid may comprise at least 99%, 98%, 97%, 96%, 95%, 94%, 93%, 92%, 91%, 90%, 89%, 88%, 87%, 86%, 85%, 84%, 83%, 82%, 81%, or 80%, or any range therein, of the total nucleic acid in the sample. For example, in a particular embodiment, the undesired nucleic acid is rRNA and comprises less than 10% of all RNA present in the sample. In another particular embodiment, the undesired nucleic acid is tRNA. In yet another particular embodiment, the undesired nucleic acid is mRNA.

Once the desired nucleic acid has been enriched, it may be further purified using methods known in the art, including, for example, commercially available purification kits such as the RNEasy MinElute column (QIAGEN, Valencia, Calif.) or the MasterPure complete DNA/RNA purification kit (Epicentre Technologies, Wis.).

In an embodiment of the invention, the enriched population of nucleic acids may be amplified by methods known in the art. Exemplary methods for DNA amplification include ligase chain reaction, strand displacement, and polymerase chain reaction (PCR), including quantitative PCR. Exemplary methods for RNA amplification include in vitro transcription, including those described in U.S. Pat. No. 7,081,340, issued Jul. 25, 2006; U.S. Pub. No. 2005/0196782, published Sep. 8, 2005; and U.S. Pub. No. 2009/0042192, published Feb. 12, 2009, all of which are hereby incorporated by reference.

In a further embodiment of the invention, the enriched population can be used to prepare cDNA according to methods known to those of ordinary skilled in the art. The cDNA may be used for subsequent manipulation and/or analysis, including, but not limited to, amplification, library preparation, sequencing, and gene expression analysis. Prior to further analysis or manipulation, the cDNA may optionally be purified by methods known in the art, including commercially available purification kits, such as the PCR MinElute column (QIAGEN, Valencia, Calif.).

The conversion of mRNA to cDNA typically involves the reverse transcription of the RNA template into cDNA, followed by its exponential amplification in a PCR reaction. The two most commonly used reverse transcriptases are avian myeloblastosis virus reverse transcriptase (AMV-RT) and Moloney murine leukemia virus reverse transcriptase (MMLV-RT). The reverse transcription step is typically primed using gene specific primers, random hexamers, or oligo-dT primers, depending on the circumstances and the goal of expression profiling. The primers may additionally contain an adaptor sequence that allows the detection or identification of the synthesized cDNA or its amplification product. Use of adaptor sequences in primers is well-known in the art. Extracted RNA can be reverse-transcribed using commercially available kits, such as the GeneAmp RNA PCR kit (Perkin Elmer, Calif., USA), following the manufacturer's instructions. The derived cDNA can then be used as a template in a subsequent PCR reaction.

Although the PCR step can use a variety of thermostable DNA-dependent DNA polymerases, it typically employs the Taq DNA polymerase, which has a 5′-3′ nuclease activity but lacks a 3′-5′ proofreading endonuclease activity. TaqMan® PCR typically utilizes the 5′-nuclease activity of Taq or Th polymerase to hydrolyze a hybridization probe bound to its target amplicon, but any enzyme with equivalent 5′ nuclease activity can be used. Two oligonucleotide primers are used to generate an amplicon typical of a PCR reaction. Again, the primers may additionally contain an adaptor sequence that allows the detection of the amplified product. A third oligonucleotide, or probe, may also be used to detect nucleotide sequence located between the two PCR primers. The probe is non-extendible by Taq DNA polymerase enzyme, and is labeled with a reporter fluorescent dye and a quencher fluorescent dye. Any laser-induced emission from the reporter dye is quenched by the quenching dye when the two dyes are located close together as they are on the probe. During the amplification reaction, the Taq DNA polymerase enzyme cleaves the probe in a template-dependent manner. The resultant probe fragments disassociate in solution, and signal from the released reporter dye is free from the quenching effect of the second fluorophore. One molecule of reporter dye is liberated for each new molecule synthesized, and detection of the unquenched reporter dye provides the basis for quantitative interpretation of the data.

TaqMan® RT-PCR can be performed using commercially available equipment, such as, for example, ABI PRISM 7700™ Sequence Detection System™ (Perkin-Elmer-Applied Biosystems, Foster City, Calif., USA), or Lightcycler (Roche Molecular Biochemicals, Mannheim, Germany). In an embodiment of the invention, the 5′ nuclease procedure is run on a real-time quantitative PCR device such as the ABI PRISM 7700™ Sequence Detection System™. The system consists of a thermocycler, laser, charge-coupled device (CCD), camera and computer. The system amplifies samples in a 96-well format on a thermocycler. During amplification, laser-induced fluorescent signal is collected in real-time through fiber optics cables for all 96 wells, and detected at the CCD. The system includes software for running the instrument and for analyzing the data.

5′-Nuclease assay data are initially expressed as Ct, or the threshold cycle. As discussed above, fluorescence values are recorded during every cycle and represent the amount of product amplified to that point in the amplification reaction. The point when the fluorescent signal is first recorded as statistically significant is the threshold cycle (Ct).

To minimize errors and the effect of sample-to-sample variation, RT-PCR is usually performed using an internal standard. The ideal internal standard is expressed at a constant level among different tissues, and is unaffected by the experimental treatment. RNAs most frequently used to normalize patterns of gene expression are mRNAs for the housekeeping genes glyceraldehyde-3-phosphate-dehydrogenase (GAPDH) and β-actin.

A variation of the RT-PCR technique is the real-time quantitative PCR, which measures PCR product accumulation through a dual-labeled fluorigenic probe (i.e., TaqMan® probe). Real time PCR is compatible both with quantitative competitive PCR, where internal competitor for each target sequence is used for normalization, and with quantitative comparative PCR using a normalization gene contained within the sample, or a housekeeping gene for RT-PCR. For further details see, e.g. Heid et al., Genome Research 6:986-994 (1996).

Microarray techniques may also be used for gene expression analysis. In this method, polynucleotide sequences of interest (such as cDNAs and oligonucleotides) are plated, or arrayed, on a microchip substrate. In a specific embodiment of the microarray technique, PCR amplified cDNA clones are applied to a substrate in a dense array. Preferably at least 10,000 nucleotide sequences are applied to the substrate. The microarrayed genes, immobilized on the microchip at 10,000 elements each, are suitable for hybridization under stringent conditions. Fluorescently labeled cDNA probes may be generated through incorporation of fluorescent nucleotides by reverse transcription of RNA extracted from tissues of interest. Labeled cDNA probes applied to the chip hybridize with specificity to each spot of DNA on the array. After stringent washing to remove non-specifically bound probes, the chip is scanned by confocal laser microscopy or by another detection method, such as a CCD camera. Quantitation of hybridization of each arrayed element allows for assessment of corresponding mRNA abundance. With dual color fluorescence, separately labeled cDNA probes generated from two sources of RNA are hybridized pairwise to the array. The relative abundance of the transcripts from the two sources corresponding to each specified gene is thus determined simultaneously. The miniaturized scale of the hybridization affords a convenient and rapid evaluation of the expression pattern for large numbers of genes. Such methods have been shown to have the sensitivity required to detect rare transcripts, which are expressed at a few copies per cell, and to reproducibly detect at least approximately two-fold differences in the expression levels (Schena et al., Proc. Natl. Acad. Sci. USA 93(2):106-149 (1996)). Microarray analysis can be performed by commercially available equipment, following manufacturer's protocols, such as by using the Affymetrix GenChip technology.

The development of microarray methods for large-scale analysis of gene expression makes it possible to search systematically for molecular markers of cancer classification and outcome prediction in a variety of tumor types. A potential benefit of RNA-depleted samples includes the application of microarray analysis to FPET samples, which have previously proven difficult. Thus, the method of the invention can be performed on a variety of nucleic acid samples including those obtained from for example, cells, tissues, organs, or lysates, fractionations, or portions thereof. In a particular embodiment, the nucleic acid sample is obtained from a fixed paraffin-embedded tissue (FPET) sample. The FPET sample may be obtained from a tumor. In an embodiment of the invention, the tumor is a cancer. In another embodiment, the cancer is selected from breast cancer, colon cancer, lung cancer, prostate cancer, hepatocellular cancer, gastric cancer, pancreatic cancer, cervical cancer, ovarian cancer, liver cancer, bladder cancer, cancer of the urinary tract, thyroid cancer, renal cancer, carcinoma, melanoma, and brain cancer.

Despite the availability of commercial products, and the extensive knowledge available concerning the isolation of RNA from tissues, isolation of nucleic acid from FPET samples and its use are not without difficulty. mRNA is often difficult to extract and maintain in its native state. Consequently, mRNA recovered from various biological sources, particularly from archived, fixed paraffin-embedded tissue (FPET), is often fragmented and/or degraded. RNA degradation progresses with archive storage time and results in RNA having an average size of about 100 bases after 17 years of storage. By comparison, intact mRNA has an average size of about 1800-2000 bases.

Typically, extraction of mRNA is followed by conversion to cDNA using the primer dependent enzyme reverse transcriptase (RT). Universal conversion of intact mRNA to cDNA is performed by oligo dT priming of the mRNA in the presence of RT. Priming with oligo dT is made possible by the presence of a poly A tract at the 3′ end of mRNA. However, it has been found that FPET mRNA is inefficiently converted to cDNA by oligo dT priming and may be due to the fact that the majority of the degraded mRNAs does not contain a polyA tail. Nevertheless, it has been shown that the degraded RNA can still be efficiently profiled for gene expression using gene-specific primers (GSP). This indicates that most regions of the expressed genes are present in the randomly fragmented RNA in proportions expected for the intact mRNA. This suggests that it should be possible to perform effective universal gene expression profiling on fragmented, e.g. FPET mRNA extracts.

To this end, it has been attempted to globally reverse transcribe FPET RNA by first polyadenylating the RNA and then performing oligo dT primed RT. Polyadenylation of FPET RNA prior to oligo dT priming increased the conversion of the RNA to cDNA by about 2-4-fold. This result has been interpreted to suggest that polyadenylation may be a useful method to prepare fragmented RNA, e.g. FPET RNA, for global reverse transcription and subsequent gene expression profiling. However, polyadenlyation can also prime other RNA, such as rRNA. Moreover, the signal amplification was still only a small fraction of that obtained by priming with gene specific primers (GSP).

Thus, in an embodiment of the invention, the method of the invention is useful for depleting undesired sequences from an FPET sample that contains a mixture of sequences, thereby enriching for a population of interest. The method comprises contacting a nucleic acid sample obtained from an FPET sample with at least one DNA probe complementary to an RNA. The DNA probe specifically hybridizes to the target RNA and forms a DNA-RNA hybrid. The sample is then contacted with an RNase H, and the RNase H degrades the RNA in the DNA-RNA hybrid. In one embodiment, the DNA probe specifically hybridizes with an rRNA and forms a DNA-rRNA hybrid. In another embodiment, the DNA probe specifically hybridizes with a tRNA and forms a DNA-tRNA hybrid. In yet another embodiment, the DNA probe specifically hybridizes with an mRNA and forms a DNA-mRNA hybrid. In a specific embodiment, the DNA probe(s) specifically hybridizes with substantially the entire full length sequence of the target RNA or to the entire full length sequence of the target RNA. Optionally, the sample may be contacted with a DNase after RNA depletion to degrade the DNA probe.

The present invention also provides compositions and kits for depleting RNA from a nucleic acid sample. As an example of a composition of the invention, the composition comprises DNA probes complementary to a target RNA. In an embodiment of the invention, the composition comprises DNA probes that specifically hybridize to substantially the entire full length sequence or to the entire full length sequence of a target RNA. In a specific embodiment, the composition comprises DNA probes of SEQ ID NOs: 1-25. In another embodiment, the composition comprises DNA probes of SEQ ID NOs: 26-88. In yet another embodiment, the composition comprises DNA probes of SEQ ID NOs: 1-88.

Any of the compositions of the invention may be comprised in a kit. For example, a composition comprising the DNA probes of SEQ ID NOs: 1-88 may be comprised in a kit. The kit will thus comprise, in a suitable container, the DNA probes of the invention. The kit may also include one or more buffers, such as a hybridization buffer, components for amplifying RNA, such as primers, as well as components for preparing a cDNA library. The kit may further include instructions for using the kit components.

The present invention is illustrated by the following Examples, which are not intended to be limiting in any way.

EXAMPLE 1 Depletion of 18s and 28s rRNA from Amplified RNA Obtained from Fixed Paraffin-Embedded Tissue (FPET) Samples of Colon Cancer Patients Samples

A single, commercially available FPET block from a colon cancer patient (ProteoGenex, Culver City, Calif.) was used as the source of RNA for all samples in this Example. Total RNA was extracted from the tissue block using the MasterPure™ Kit (Epicentre Biotechnologies, Madison, Wis.). The total RNA was amplified as outlined in FIG. 2 and described in U.S. Pub. No. 2005/0196782, published Sep. 8, 2005, and U.S. Pub. No. 2009/0042192, published Feb. 12, 2009. Briefly, either 25 ng or 50 ng of an RNA sample was converted to cDNA via reverse transcription using oligo dT and random nonamers. The RNA was then degraded by incubation with RNase H and the cDNA was poly-dT tailed through the action of a terminal deoxynucleotidyl transferase (TdT) enzyme. A T7 RNA polymerase promoter sequence was then hybridized to the tailed cDNA, and made double-stranded through the activity of DNA polymerase. This double-stranded cDNA was then used as a template for in vitro transcription. The process generally yields as much as 100 μg of amplified RNA per sample. This amplified RNA maintains a high degree of similarity with the original source material RNA, but contains a T7 RNA polymerase promoter sequence appended at the end of each strand. Thus, the amplified RNA can be used as a surrogate for the source material.

Depletion of 18S and 28S rRNA from the Amplified RNA Sample

The amplified RNA was depleted of 18S and 28S rRNA, with or without further DNAse I treatment. Four different samples were generated and are described in Table 2:

TABLE 2 # Sample Name Description 1 Not Depleted Amplified RNA, not depleted 2 Depleted Method1 Amplified RNA, depleted, no DNAse I 3 Depleted Method2 Amplified RNA, depleted, DNAse I-treated 4 Depleted Method2 rep Replicate of #3

One microgram of the amplified RNA obtained was mixed with an equimolar mixture of the eighty-eight synthetic DNA probes shown in Table 1. Taken together, the 88 DNA probes represent the entire full length complementary sequence of human 18s rRNA (GenBank accession M10098) and human 28s rRNA (GenBank accession M11167). The mixture was heated and then cooled in a manner that produces double-stranded DNA-RNA hybrids. The mixture was then incubated with RNAse H under appropriate buffer components at a defined temperature for a defined period of time. The mixture was then incubated with DNAse I under appropriate buffer components at a defined temperature for a defined period of time. The mixture was then purified to remove protein from the RNA preparations. The procedure is outlined as follows:

1. Denature and slow-cool RNA/probe mix

-   -   a. In one 200 uL PCR tube, the following were added:         -   1 microgram total RNA from FPET         -   937 nanograms rRNA antisense DNA probe pool         -   1 μL 5× Hybridization Buffer (500 mM Trizma-HCl; 1M NaCl)

The mixture was brought to a final volume of 5 μL with nuclease-free water.

-   -   b. The mixture was incubated as follows using a thermocycler         with a heated lid:         -   95° C. for 2 minutes         -   Ramp to 22° C. at 0.1° C./second         -   Place on ice

2. RNAse H Digestion

-   -   a. The following were added to the RNA/probe mix from step 1.b         above:         -   10 units Hybridase™ (Epicentre, Madison, Wis.) RNAse H         -   1 μL 10× Hybridase Buffer 2 (500 mM Trizma-HCl; 1M NaCl; 200             mM MgCl₂)

The mixture was brought to a final volume of 10 μL with nuclease-free water.

-   -   b. The mixture was incubated as follows using a thermocycler         with a heated lid:         -   37° C. for 30 minutes         -   Place on ice     -   c. The sample was purified using an RNEasy MinElute column         (QIAGEN, Valencia, Calif.) according to manufacturer's         instructions. The purified sample had a final volume of         approximately 12 μL.

3. DNAseI Treatment

-   -   a. The following were added to the purified RNA in step 2.c:         -   4 units DNAseI (NEB, Ipswich, Mass.)         -   2 uL 10× NEB Buffer

Nuclease-free water was added to bring the volume to 20 μL

-   -   b. The mixture was incubated as follows using a thermocycler         with a heated lid:         -   37° C. for 30 minutes         -   Place on ice     -   c. The sample was purified using an RNEasy MinElute column         (QIAGEN, Valencia, Calif.) according to manufacturer's         instructions. The purified sample had a final volume of         approximately 12 μL.

RT-PCR was performed on the samples to determine relative abundance of the 18S and 28S rRNA. FIG. 3 shows the relative rRNA depletion in samples that were mock treated (Sample #1; blank boxes); treated with RNAse H (Sample #2; filled boxes); or treated with RNAse H followed by DNAse I treatment (Sample #3; hatched boxes). RT-PCR of three different regions of 18S and 28S was performed in triplicate on each sample. β-actin and RPL were used as controls. RNAse H treatment, with or without subsequent DNAse I treatment, reduced the abundance of 18S and 28S rRNA from the samples, while the mock-treated sample showed little or no reduction in the rRNAs. After RNAse H treatment, the 18S and 28S comprised an average of 7% of total RNA in the sample. After RNAse H and DNAse I treatment, the 18S and 28S comprised an average of 4% of total RNA in the sample. RNAse H and DNAse I treatments had little effect on the control genes.

To demonstrate that rRNA depletion does not alter the gene expression profiles, correlation graphs were prepared using average difference values for 93 or 94 genes in amplified RNA and rRNA-depleted amplified RNA. A perfect correlation between different samples would be indicated by an r² value of 1. FIG. 4 shows the average difference correlation comparing the results of amplified RNA without rRNA depletion (Sample #1; “Amplified RNA” in the x-axis) and rRNA-depleted amplified RNA without DNAse I treatment (Sample #2; “Depleted Method1 RNA” in the y-axis) over 96 genes. Genes that did not meet the detection threshold were excluded. The r² value in this case was 0.983 over 94 genes, indicating a good correlation between the two samples.

FIG. 5 shows the average difference correlation comparing the results of amplified RNA without rRNA depletion (Sample #1; “Amplified RNA” in the x-axis) and rRNA-depleted amplified RNA with DNAse I treatment (Sample #3; “Depleted Method2 RNA” in the y-axis) over 93 detectable genes. Again, the r² value in this case was 0.987, indicating a good correlation between the two samples.

EXAMPLE 2 Preparation of cDNA Libraries from rRNA-Depleted Amplified RNA Obtained from FPET

As illustrated in FIG. 6, the amplified RNA from Example 1 was reverse-transcribed with adaptor (P1)-modified random hexamers (P1-N6 primer) (P1: 5′-AGATCGGAAGAGCGTCGTGTAGGGAAAGAGTGTAGATCTCGGTGGTCGCC GTATCATT-3′) (SEQ ID NO: 89) to produce cDNA. The cDNA was resolved on a 6% TBE-Urea gel and the region from 116-200 bp was excised and cut into four equal slices. PCR was performed on one slice using the P1 primer and an adaptor (P2)-modified T7 primer (P2-T7 primer) containing an intervening nucleotide “barcode” between the P2 and T7 sequences that serves to identify each sample (P2: 5′-CAAGCAGAAGACGGCA TACGAGAT-3′) (SEQ ID NO: 90); (T7: 5′-GGGAGACGCGTGTAAA-3′) (SEQ ID NO: 91). The PCR reaction was then purified. The procedure is outlined below.

1. First-strand cDNA synthesis

-   -   a. Primer hybridization. The following were mixed:         -   500 ng amplified, rRNA-depleted RNA         -   2 μL of a 2 uM solution of P1-N6 primer         -   Brought to 5 μL with nuclease-free water.     -   b. The mixture was incubated as follows using a thermocycler         with a heated lid:         -   37° C. for 10 minutes         -   Place on ice     -   c. Reverse Transcription. The following were mixed:         -   2 μL 5×RT Buffer (Invitrogen, Carlsbad, Calif.)         -   1 μL DTT (20 mM)         -   1 μL dNTP mix (10 mM each)         -   1 μL SuperScript II™ (Invitrogen, Carlsbad, Calif.)         -   5 μL RNA/primer mix from step 1.b     -   d. The mixture was incubated as follows using a thermocycler         with a heated lid:         -   20° C. for 10 minutes         -   37° C. for 10 minutes         -   42° C. for 45 minutes         -   96° C. for 5 minutes         -   Place on ice

2. cDNA Purification/Size Selection

-   -   a. The cDNA reaction from step 1.d was purified using a PCR         MinElute column (QIAGEN, Valencia, Calif.) according to         manufacturer's instructions. The final eluate volume was 10 μL.     -   b. 5 μL of the sample from step 2.a was mixed with 5 μL 2× gel         loading buffer. The mixture was incubated as follows using a         thermocycler with a heated lid:         -   95° C. for 3 minutes         -   Place on ice     -   c. The reaction from step 2.b was loaded onto one lane of a 1 mm         thick 6% TBE-Urea gel and run for 17 minutes at 180 Volts or         until the leading dye front reached the middle of the gel.         Molecular weight ladders were also run in adjacent lanes. The         gel was stained in 1:10,000 SYBR-Gold for 10 minutes.     -   d. Using a razor blade, the portion of the lane containing the         sample that corresponded to a molecular weight of from 116-200         nt was excised and further vertically subdivided into four         approximately equal slices.

3. PCR Amplification

-   -   a. Each slice was mixed with the following:         -   2 μL dNTP mix (10 mM ea.)         -   10 μL 10×PCR Advantage™ (Clontech, Mountain View, Calif.)             Buffer         -   3 μL of an 8 uM solution of P1-N6 PCR primer         -   3 μL of an 8 uM solution of P2-T7 PCR primer         -   2 μL Advantage™ (Clontech, Mountain View, Calif.) DNA             polymerase         -   80 μL nuclease-free water     -   b. The mixture was incubated as follows using a thermocycler         with a heated lid:     -   1 cycle         -   94° C. for 1 minute     -   2 cycles         -   94° C. for 15 seconds         -   55° C. for 30 seconds         -   72° C. for 30 seconds     -   12 cycles         -   94° C. for 15 seconds         -   62° C. for 30 seconds         -   72° C. for 30 seconds     -   1 cycle         -   72° C. for 7 minutes     -   c. cDNA was purified using a PureLink™ PCR Micro Purification         Kit (Invitrogen, Carlsbad, Calif.) according to manufacturer's         instructions.

Table 3 shows the yield of the cDNA libraries prepared from each amplified RNA sample of Table 2.

TABLE 3 Yield (ng)* Depleted Not Depleted Depleted Method2 Depleted Method1 Method2 rep 732 308 848 877 *Based on using half the size-selected RT rxn. Potential yield is twice what is shown.

Table 4 shows the insert lengths of a number of clones analyzed (n) from the cDNA libraries prepared from each amplified RNA sample of Table 2.

TABLE 4 Insert length** Depleted Not Depleted Depleted Method2 Depleted Method1 Method2 rep Avg 82.9 77.8 74.2 60.2 Median 81 90 79 73 Max 122 139 109 116 Min 35 6 19 5 n 13 20 18 25 # <50 bp 1 4 3 9 **Excludes the TdT tail, present at the 5′-end of all amplified RNA transcipts. Inserts of 50-134 bp were targeted.

As shown in FIGS. 4 and 5, the average difference correlation between amplified RNA without rRNA-depletion (Sample #1) and amplified RNA with rRNA-depletion (Sample #2) or rRNA-depleted amplified RNA with DNAse I treatment (Sample #3) exhibited good correlation (r²=0.983 and 0.987, respectively). The correlation between the rRNA-undepleted and -depleted samples indicates that the gene expression profiles were not altered by the rRNA depletion step. Average difference correlations were also determined in the cDNA libraries prepared from the samples shown in Table 2. Results are shown in Table 5.

As shown in Table 5, the average difference correlations between amplified RNA before cDNA library preparation and amplified RNA after library preparation exhibited low correlation, indicating that the gene expression profile is different in the cDNA library preparation compared to the starting material. Without being bound to theory, this may be due to the possibility that the cDNA insert size in the library is smaller than the region that some of the PCR primers are able to bind. However, the alterations were consistent and reproducible across the library preparations.

Clones from each cDNA library were also sequenced. The results are shown in Table 6. In the cDNA library prepared from rRNA-undepleted RNA (Sample #1), 13 clones were sequenced, of which seven mapped to the 18S/28S rRNA, and six mapped to mitochondrial genes. Thus, 54% of the clones sequenced from the rRNA-undepleted library constituted 18S/28S rRNA. In contrast, approximately 4% or less of the clones sequenced from the rRNA-depleted libraries mapped to 18S/28S rRNAs. Moreover, there was an increase in the percentage of clones that mapped to known genes (“refseq”: 30-56%).

TABLE 6 Depleted Method2 Not Depleted Depleted Method1 Depleted Method2 Replicate Sequencing Summary # Clones % # Clones % # Clones % # Clones % mapped to refseq 0   0% 6  30% 10 56% 8  32% mapped to genome 0   0% 0   0% 1  6% 0   0% 18s/28s rRNA 7  54% 0   0% 0  0% 1   4% 5s rRNA 0   0% 1   5% 1  6% 0   0% Mitochondrial 6  46% 9  45% 6 33% 10  40% not mapped/too short 0   0% 1   5% 0  0% 4  16% poor sequence quality 0   0% 3  15% 0  0% 2   8% Total 13 100% 20 100% 18 1 25 100% Method 1: Hybridase ™ Method 2: Hybridase ™ + DNAse Targeted for depletion: 18s/28s rRNA

EXAMPLE 3 Depletion of 18s and 28s rRNA from Amplified RNA Obtained from Fixed Paraffin-Embedded Tissue (FPET) Samples of Breast Cancer Patients Samples

Two breast cancer FPET blocks were obtained from Providence—St. Joseph Medical Center, Burbank Calif., and were used as the source of RNA for all samples in this Example. Total RNA was extracted from the tissue block using the MasterPure™ Kit (Epicentre Biotechnologies, Madison, Wis.) and the total RNA was amplified as described in Example 1. The RNAs were classified as “Lo ER” (exhibiting low EstR1 gene expression) or “Hi ER” (exhibiting high EstR1 gene expression).

FIGS. 7 and 8 show the average difference correlation of total unamplified RNA vs amplified Lo ER RNA or amplified Hi ER RNA, respectively, over 96 genes. Genes that did not meet the detection threshold were excluded. The r² values were 0.876 and 0.891, respectively.

Depletion of 18S and 28S rRNA from the Amplified RNA Sample

The “Lo ER” and “Hi ER” amplified RNA were mixed in different ratios to create additional samples for library preparation. The amplified RNAs were depleted of 18S and 28S rRNA and all samples were treated with DNAse I. The following eight amplified RNA samples were generated:

TABLE 7 # Sample Name Description 1 Lo ER n = 1 Lo ER amplified RNA, DNAse I treated 2 Lo ER n = 2 Replicate of #1 3 Hi ER n = 1 Hi ER amplified RNA, DNAse I treated 4 Hi ER n = 2 Replicate of #3 5 Lo/Hi 50/50 n = 1 Mixed amplified RNA, DNAse I treated 6 Lo/Hi 50/50 n = 2 Replicate of #5 7 Lo/Hi 75/25 n = 1 Mixed amplified RNA, DNAse I treated 8 Lo/Hi 75/25 n = 2 Replicate of #7

The amplified RNAs were depleted for 18S and 28S according to Example 1. All of the amplified RNAs were also treated with DNAse I using the RNAse-free DNAse Set (Qiagen, Valencia, Calif.). After step 2b in Example 1, 10 μl of each amplified RNA was mixed with 77.5 μl of H₂O, 10 μl Buffer RDD, and 2.5 μl of the DNAse I stock and incubated for 10 minutes at room temperature. The samples were then purified using an RNEasy MinElute column (QIAGEN, Valencia, Calif.) according to manufacturer's instructions. The purified sample had a final volume of approximately 12 μL.

RT-PCR was performed for three different regions of 18S and 28S in triplicate on each type of sample (Lo ER, Hi ER, Lo/Hi 50/50, and Lo/Hi 75/25) to determine relative abundance of the 18S and 28S rRNA before and after DNAse I treatment. Relative rRNA depletion of the 4 samples was averaged. FIG. 9 shows the relative rRNA depletion in untreated samples (blank boxes); treated with RNAse H (filled boxes); or treated with RNAse H followed by DNAse I treatment (hatched boxes). β-actin and RPL were used as controls. RNAse H treatment, with or without subsequent DNAse I treatment, reduced the abundance of 18S and 28S rRNA from the samples, while the untreated sample showed little or no reduction in the rRNAs. After RNAse H treatment, the 18S and 28S comprised an average of 4.3% of total RNA in the sample. After RNAse H and DNAse I treatment, the 18S and 28S comprised an average of 3.0% of total RNA in the sample. RNAse H and DNAse I treatments had little effect on the control genes.

To demonstrate that rRNA depletion does not alter the gene expression profiles, correlation graphs were prepared using average difference values for 96 genes in amplified RNA and rRNA-depleted amplified RNA. A perfect correlation between different samples would be indicated by an r² value of 1. FIG. 10 shows the average difference correlation comparing the results of rRNA-undepleted Lo ER amplified RNA (“Amplified” in the x-axis) and rRNA-depleted Lo ER amplified RNA with DNAse I treatment (Sample #1; “Depleted” in the y-axis) over 96 genes. Genes that did not meet the detection threshold were excluded. The r² value in this case was 0.940, indicating a good correlation between the two samples.

FIG. 11 shows the average difference correlation comparing the results of rRNA-undepleted Hi ER amplified RNA (“Amplified” in the x-axis) and rRNA-depleted Hi ER amplified RNA with DNAse I treatment (Sample #3; “Depleted” in the y-axis) over 96 genes. Genes that did not meet the detection threshold were excluded. The r² value in this case was 0.922, indicating a good correlation between the two samples.

FIG. 12 shows the average difference correlation comparing the results of duplicates of rRNA-depleted Hi ER amplified RNA (Sample #3 and #4; “Depleted n=1” in the x-axis and “Depleted n=2” in the y-axis, respectively) over 96 genes. Genes that did not meet the detection threshold were excluded. The r² value in this case was 0.962, indicating a good correlation between the duplicate samples.

EXAMPLE 4 Preparation of cDNA Libraries from rRNA-Depleted Amplified RNA Obtained from FPET

cDNA libraries were prepared from the rRNA-depleted amplified RNAs of Example 3 according to the method set forth in Example 2, except that that in step 2d, gel bands corresponding to molecular weights of from 130-180 nt were excised, instead of 116-200 nt. The size distribution of the cDNA library preparations were compared to that of a readily available cDNA library (Illumina “Preparing Samples for Sequencing of mRNA” Manual, FIG. 1) and were found to be similar (FIG. 13).

The amount of nucleic acid was measured using a picogreen fluorescent assay (PG). The yield of the cDNA library from each sample is shown in Table 8.

TABLE 8 Avg PG conc. Insert nM nM PG Yield Description (ng/uL) (bp) (Agilent) (calc'd) (ng) Lo ER n = 1 96.4 182.0 594 802 1927.5 Lo ER n = 2 99.6 181.0 745 834 1991.6 Hi ER n = 1 75.5 182.0 577 628 1509.9 Hi ER n = 2 97.7 183.0 596 809 1953.3 Lo/Hi 50/50 n = 1 87.0 182.0 574 724 1739.6 Lo/Hi 50/50 n = 2 87.2 178.0 637 742 1744.1 Lo/Hi 75/25 n = 1 62.7 178.0 462 534 1254.9 Lo/Hi 75/25 n = 2 101.6 175.0 672 800 2033.0

As summarized in Table 9, the average difference correlation determined for 96 genes between rRNA-depleted amplified RNAs (Lo ER n=1; LoER n=2; Hi ER n=1; and Hi ER n=2) and their source material (total RNA) ranged from r²=0.853 to 0.894. The rRNA-depleted amplified RNAs showed good correlation to amplified RNA without rRNA depletion, where r² ranged from 0.905 to 0.947. Replicate experiments of amplified RNA also showed a good correlation, where r² ranged from 0.934 to 0.962. The rRNA-depleted amplified RNA contained between 2.6% to 4.8% rRNA.

Average difference correlations were also determined for 96 genes in the cDNA libraries prepared from each of the samples shown in Table 7. Genes that did not meet the detection threshold were excluded. As shown in Table 9, the average difference correlations between rRNA-depleted amplified RNA before cDNA library preparation (“Depleted” column in Table 9) and rRNA-depleted amplified RNA after library preparation exhibited r² values ranging from 0.664 to 0.748, indicating that the gene expression profile is different in the cDNA library preparation compared to the starting material. Again, without being bound to theory, the difference may be due to the possibility that the cDNA insert size in the library is smaller than that detectable by the PCR primers used. However, replicates of each cDNA library preparation showed good correlations, where r² ranged from 0.830 to 0.925, indicating the reproducibility of the library preparation process.

TABLE 9 r² Values for <96 genes Depleted RNAs Libraries Description Source Amplified Replicate % rRNA Source Amplified Depleted Replicate Lo ER n = 1 0.875 0.940 0.934 3.9% 0.573 0.689 0.704 0.881 Lo ER n = 2 0.853 0.933 3.0% 0.616 0.732 0.698 Hi ER n = 1 0.886 0.922 0.962 4.8% 0.662 0.764 0.748 0.925 Hi ER n = 2 0.894 0.947 3.3% 0.645 0.749 0.740 Lo/Hi 50/50 n = 1 NA 0.905 0.961 4.3% NA 0.735 0.727 0.901 Lo/Hi 50/50 n = 2 NA 0.942 3.2% NA 0.694 0.696 Lo/Hi 75/25 n = 1 NA 0.905 0.951 4.2% NA 0.681 0.664 0.830 Lo/Hi 75/25 n = 2 NA 0.947 2.6% NA 0.670 0.679

Clones from each cDNA library were sequenced by the traditional Sanger sequencing method in two batches. The cDNA inserts in the library had the structure as illustrated in FIG. 14. In Batch 1, a total of 90 clones from cDNA library preparations from Lo ER n=1; Hi ER n=1; Lo/Hi 50/50 n=1; and Lo/Hi 75/25 n=1 were sequenced, and in Batch 2, a total of 92 clones from cDNA library preparations from Lo ER n=2; Hi ER n=2; Lo/Hi 50/50 n=2; and Lo/Hi 75/25 n=2 were sequenced. The results are shown in Table 10. 42-44% of the clones contained inserts of greater than 50 bp, while the average insert size was between 43-45 bp.

TABLE 10 Sequence Data Batch1 Batch2 n % n % # clones 90 92 >=50 bp 40 44 39 42 Length (bp) Avg 43 45 insert Avg 48 49 polyT Targeted insert size: 50-100 bp Assumed polyT was: 30 bp cDNA selection range: 130-180 b Predicted PCR product range: 178-228 bp Observed PCR product range: 130-230 bp

The transcripts of 81 clones from Batch 1 (“n=1”) and 82 clones from Batch 2 (“n=2”) were analyzed. Of the 81 clones from Batch 1, 21 clones were from the Lo ER n=1 sample; 17 clones were from the Hi ER n=1 sample; 19 clones were from the Lo/Hi 50/50 n=1 sample; and 15 clones were from the Lo/Hi 75/25 n=1 sample. 8 clones from Batch 1 were classified as “No Class” because they could not be associated with a specific sample type as the “barcode” region was unreadable. Of the 82 clones from Batch 2, 18 clones were from the Lo ER n=2 sample; 17 clones were from the Hi ER n=2 sample; 19 clones were from the Lo/Hi 50/50 n=2 sample; and 12 clones were from the Lo/Hi ER 75/25 n=2 sample. 15 clones from Batch 2 were classified as “No Class” because they could not be associated with a sample type as the “barcode” region was unreadable. The transcript distributions from each sample and from each batch are shown in Table 11. The percentage of clones that mapped to 18S/28S rRNAs ranged from 0% to 17.6% across all samples, with the exclusion of Hi ER n=2, which exhibited the highest at 35.3%. The total average of rRNA in the samples was 13%, which may be slightly skewed due to the high percentage of rRNA seen in the Hi ER n=2 sample. The percentage of clones that mapped to known genes (refseq/genomic) ranged from 33.3% to 57.9%, for a total average of 43.5%.

TABLE 11 Lo/Hi Lo/Hi Lo/Hi Lo/Hi Lo ER Lo ER Hi ER Hi ER 50/50 50/50 75/25 75/25 % on Sub-Total n = 1 n = 2 n = 1 n = 2 n = 1 n = 2 n = 1 n = 2 refseq/genomic  52.4%  55.6%  41.2%  41.2%  57.9% 42.1% 40.0% 33.3% 18s/28s   4.8%  11.1%  17.6%  35.3%  10.5%  5.3%  6.7%  0.0% 5s/mito   4.8%  11.1%   5.9%  17.6%   5.3% 36.8% 33.3% 16.7% not mapped  38.1%  22.2%  35.3%   5.9%  26.3% 15.8% 20.0% 50.0% Total mapped  61.9%  77.8%  64.7%  94.1%  73.7% 84.2% 80.0% 50.0% Total 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% No Class No Class Sub-Total Sub-Total % on Sub-Total N = 1 N = 2 N = 1 N = 2 Total refseq/genomic  25.0%  26.7%  46.3%  40.7%  43.5% 18s/28s  12.5%  26.7%  10.0%  16.0%  13.0% 5s/mito   0.0%   0.0%  10.0%  17.3%  13.7% not mapped  62.5%  46.7%  33.8%  25.9%  29.8% Total mapped  37.5%  53.3%  66.3%  74.1%  70.2% Total 100.0% 100.0% 100.0% 100.0% 100.0%

Each of the eight libraries was also sequenced on the Illumina Genome Analyzer IIx. On average, 14 million reads were obtained from each library with the exception of one library (sample #2). Inserts of 20 bp-50 bp were further analyzed.

As can be seen from Table 12, approximately 80% or more of the inserts having 20 bp-50 bp (“≥20 bp inserts”) mapped to the human genome. Moreover, approximately 37%-47% of these inserts mapped to unique sequences in the genome (“UniqMapped”). Furthermore, when these uniquely mapped sequences from samples 1 and 3 were compared against the Reference Sequence (RefSeq) database (NCBI), 57% and 57.9% of the sequences, respectively, mapped to sequences contained in the RefSeq database. This indicates that approximately 57% of the sequences mapped to known coding sequences and that the remaining 43% of the sequences may represent non-coding transcripts.

TABLE 12 ≥ 20 bp inserts mapped Multi + Multi + Uniq ≥20 bp to 20-35 bp uniq Uniq 36-50 bp uniq Mapped inserts genome Sample total reads inserts Mapped Mapped inserts Mapped (%) mapped (%) 1 14476625 2263253 1461741 930867 10066382 5130706 4052005 12329635 85.16 Lo ER n = 1 (64.59%) (41.13%) (50.97%) (40.25%) 2 2929038 437840 305514 198191 2015051 1193357 948534 2452891 83.7 Lo ER n = 2 (69.78%) (45.27%) (59.22%) (47.07%) 3 18546600 2845649 1933907 1252440 12582885 7127512 5593700 15428534 83.1 Hi ER n = 1 (67.96%) (44.01%) (56.64%) (44.45%) 4 12188981 2067725 1361192 872608 7785415 3918539 3067250 9853140 80.8 Hi ER n = 2 (65.83%) (42.20%) (50.33%) (39.40%) 5 16739798 2720576 1769961 1137848 11287538 5711278 4513236 14008114 83.68 Lo/Hi 50/50 (65.06%) (41.82%) (50.60%) (39.98%) n = 1 6 15381054 2517832 1727462 1118627 10121556 5592295 4341020 12639388 82.17 Lo/Hi 50/50 (68.61%) (44.43%) (55.25%) (42.89%) n = 2 7 18389310 3229602 2156802 1396370 11560179 5823826 4546313 14789781 80.42 Lo/Hi 75/25 (66.78%) (43.24%) (50.38%) (39.33%) n = 1 8 15507054 3127318 2057227 1317400 9249961 4447856 3442283 12377279 79.81 Lo/Hi 75/25 (65.78%) (42.13%) (48.09%) (37.21%) n = 2

Inserts of 20 bp-50 bp were also analyzed for 18S, 28S, and 5.8S rRNA sequences, as well as for mitochondrial genes (“mito”). As can be seen from Table 13, the 18S, 28S, and 5.8S rRNA constituted approximately less than 10% the total reads (“rRNA % total”) or total inserts that were analyzed (“rRNA % ≥20 bp”). However, 5.8S rRNA had not been depleted from the samples. Thus, when 5.8S rRNA is excluded from these numbers, 18S and 28S rRNA constituted less than 5% of the total reads.

TABLE 13 total Sample reads 20-35bp 18S 28S 5.8S mito rRNA 1 14476625 2263253 100949 190219 22753 116695 313921 Lo ER n = 1 2 2929038 437840 18428 33913 4043 42484 56384 Lo ER n = 2 3 18546600 2845649 109101 191727 28572 241756 329400 Hi ER n = 1 4 12188981 2067725 69919 122876 22498 137012 215293 Hi ER n = 2 5 16739798 2720576 98527 173084 30887 151119 302498 Lo/Hi 50/50 n = 1 6 15381054 2517832 95160 171444 26711 247515 293315 Lo/Hi 50/50 n = 2 7 18389310 3229602 104324 181535 35025 249993 320884 Lo/Hi 75/25 n = 1 8 15507054 3127318 115144 208282 35155 202777 358581 Lo/Hi 75/25 n = 2 rRNA rRNA % total % ≥20 Sample 36-50 bp 18S 28S 5.8S mito rRNA rRNA total bp 1 10066382 244768 350460 202973 388988 798201 1112122 7.68 9.01 Lo ER n = 1 2 2015051 43794 52779 42772 172268 139345 195729 6.68 7.97 Lo ER n = 2 3 12582885 198595 250177 286053 906700 734825 1064225 5.73 6.89 Hi ER n = 1 4 7785415 73362 105380 167218 391182 345960 561253 4.60 5.69 Hi ER n = 2 5 11287538 131659 161722 245841 454534 539222 841720 5.02 6.00 Lo/Hi 50/50 n = 1 6 10121556 158350 186692 242832 242832 587874 881189 5.72 6.97 Lo/Hi 50/50 n = 2 7 11560179 94540 119614 238378 679351 452532 773416 4.20 5.22 Lo/Hi 75/25 n = 1 8 9249961 135103 169811 187973 433510 492887 851468 5.49 6.87 Lo/Hi 75/25 n = 2

EXAMPLE 5 Preparation of cDNA Libraries Using rRNA-Depleted RNA Obtained from FPET and Not Subjected to Amplification

The FPET RNA used in this example is comprised of a pool of RNAs extracted from patient breast tumor biopsy samples submitted to Genomic Health®, Inc. for use in the OncoTypeDX® Breast Cancer Assay. This RNA pool was not subjected to the amplification process as described in Example 2 and is referred to hereafter as the commercial RNA pool.

A first cDNA library was prepared from one microgram of the commercial RNA pool using the mRNA-Seq Kit™ (Illumina®, San Diego, Calif.) and is here referred to as the “undepleted library.” A second cDNA library was prepared using the same Illumina® kit using one microgram of the commercial RNA pool, except that the RNA was subjected to rRNA-depletion as described in Example 1 before submitting it to library preparation and is here referred to as the “depleted library.” Both libraries were prepared according to the mRNA Sequencing Sample Preparation Guide (part# 1004898 Rev. D, Illumina®, San Diego, Calif.) with the following modifications: The fragmentation and polyA+ selection steps were omitted; random hexamers and enzyme reagents were obtained from NEB (Ipswich, Mass.); adapter oligos and PCR primers were obtained from Integrated DNA Technologies (Coralville, Iowa); size-selection at 250-300 bp was performed on a Pippen Prep™ (Sage Science, Beverly, Mass.). Including modifications as noted above, the method comprised the following steps: First-strand cDNA synthesis, second-strand cDNA synthesis, end-repair, adenylation, ligation, size-selection and PCR amplification. The size of each library was determined by DNA 1000 Assay™ (Agilent, Santa Clara, Calif.) to be approximately 300-330 bp, which is consistent with a 250-300 bp size selection post-ligation plus additional sequences added during the PCR step. Library yields were determined by PicoGreen™ Assay (Invitrogen, Carlsbad, Calif.). The yield of the undepleted library was determined to be 300.3 ng. The yield of the depleted library was determined to be 894.7 ng.

The undepleted and depleted libraries were sequenced on separate lanes of a single flow cell on the Genome Analyzer IIx (Illumina, San Diego, Calif.) using single-read chemistry with a fifty-one base-pair read length. Read yields were 28 million and 21 million for the undepleted and depleted libraries, respectively (Table 14).

TABLE 14 Targeted rRNA Reads Other 18S + 28S 5.8S rRNA Total rRNA Library 18S Reads 28S Reads Reads Reads Reads Reads Total Reads undepleted 3,935,743 8,539,112 12,474,855 3,505 29,439 24,982,654 28,025,533 commercial pool depleted 95,780 475,912 571,692 354,665 557,210 2,055,259 20,778,050 commercial pool

TABLE 15 Targeted rRNA % 18S % Unique Mapped % + Other Total RefSeq RefSeq + Library 18S % 28S % 28S % 5.8S % rRNA % rRNA % Only rRNA undepleted commercial pool  14.0%  30.5%  44.5%   0.0%   0.1%  44.6% 31.7%  76.9% depleted commercial pool   0.5%   2.3%   2.8%   1.7%   2.7%   7.1% 61.2%  67.8% % change −96.7% −92.5% −93.8% 13548.3% 2453.0% −84.0% 92.8% −11.8%

As can be seen from Table 15, 68% and 77% of 51 bp inserts for the depleted and undepleted libraries, respectively, uniquely mapped to the Reference Sequence (RefSeq) Database (NCBI) when rRNA sequences are included in the database. However, when rRNA sequences are excluded from the database, only 31.7% of undepleted sequences uniquely map to the RefSeq transcripts, as compared to 61.2% for the depleted library. This observation illustrates the principal benefit of rRNA depletion; namely, the corresponding increase in the representation of RefSeq transcripts in the sequence count data.

The “Targeted rRNA” in Tables 14 and 15 refer to 18S and 28S rRNA. 18S and 28S rRNAs typically comprise 99.7% of the total rRNA of the undepleted library, illustrating the usefulness of removing these sequences. Compared to the undepleted library, the Targeted rRNAs in the depleted library were reduced by 93.8%. However, Total rRNA is reduced by only 84%, which is due to an increase in the sampling rate of rare rRNA species (e.g., 5.8S) after the more abundant 18S and 28S species have been removed.

EXAMPLE 6 Historical Performance of rRNA Depletion Based on Analysis of RNA-Seq Count Data Generated on the Illumina GAIIx Sequencing Platform

Table 16 summarizes count data for fifty-five separate rRNA-depleted libraries that were generated over a period of six months. The RNAs that serve as the source material for these libraries include the commercial RNA pool described in Example 5 (Table 16, sample ID 3) and commercial mammary gland total RNA (Ambion, Austin, Tex., Table 16, sample ID 4), as well as RNAs extracted from individual patient tumor or normal breast biopsy FPET specimens obtained from Marin General Hospital (Greenbrae, Calif.) using a High Pure miRNA Kit™ (Roche, Mannheim, Germany). Preparation of cDNA libraries and generation of sequence data on the Genome Analyzer IIx (Illumina, San Diego, Calif.) is as described in Example 5. Total read yield was in the range of 18 million to 38 million reads for each of the libraries listed in Table 16. Table 16 summarizes the proportion of total reads for each library that map to various rRNA species.

TABLE 16 Sample FPET 5.8S 18S Other Total ID Tissue FPET? Block Age % % 28S % rRNA % rRNA % 1 Breast tumor pool yes 10 yr.  8.4% 0.4%  3.1% 2.8% 14.8% 1 Breast tumor pool yes 10 yr.  6.0% 0.2%  1.6% 2.0%  9.9% 2 Breast normal pool yes 3 mos.  5.2% 1.7%  6.1% 0.9% 13.9% 2 Breast normal pool yes 3 mos.  6.6% 1.6%  5.9% 1.2% 15.2% 3 Breast tumor pool yes 1 month 35.9% 0.1%  0.3% 1.1% 37.4% 3 Breast tumor pool yes 1 month 25.7% 0.0%  0.1% 1.5% 27.4% 3 Breast tumor pool yes 1 month 32.9% 0.1%  0.2% 1.1% 34.4% 3 Breast tumor pool yes 1 month 12.5% 0.2%  0.3% 2.0% 15.1% 3 Breast tumor pool yes 1 month  2.1% 0.1%  0.1% 2.4%  4.8% 3 Breast tumor pool yes 1 month  0.9% 0.0%  0.1% 2.5%  3.5% 3 Breast tumor pool yes 1 month  2.2% 0.1%  0.1% 0.7%  3.1% 4 Breast normal no NA  4.1% 4.2%  6.9% 1.6% 16.8% 4 Breast normal no NA  5.9% 3.5%  5.3% 2.0% 16.8% 3 Breast tumor pool yes 1 month  9.3% 0.1%  0.2% 2.0% 11.6% 3 Breast tumor pool yes 1 month  6.6% 0.0%  0.2% 2.1%  8.8% 5 Breast tumor single patient yes 10 yr.  2.1% 0.1%  0.3% 1.6%  4.1% 5 Breast tumor single patient yes 10 yr.  1.9% 0.1%  0.3% 1.4%  3.7% 6 Breast Normal single patient yes 10 yr.  3.8% 3.8%  0.7% 0.8%  9.2% 7 Breast Normal single patient yes 10 yr.  4.2% 0.1%  1.0% 1.6%  6.9% 8 Breast Normal single patient yes 10 yr.  2.1% 0.4%  3.2% 1.2%  6.9% 9 Breast Normal single patient yes 10 yr.  0.8% 1.5%  9.1% 1.0% 12.3% 10 Breast Normal single patient yes 10 yr.  1.1% 0.1%  0.8% 0.8%  2.8% 11 Breast Normal single patient yes 10 yr.  0.8% 5.0% 17.6% 0.6% 23.8% 12 Breast Normal single patient yes 10 yr.  1.8% 3.2% 15.1% 0.9% 21.1% 13 Breast Normal single patient yes 10 yr.  0.3% 0.6%  6.1% 0.7%  7.7% 14 Breast Tumor single patient yes 10 yr.  4.1% 0.0%  0.1% 1.4%  5.6% 15 Breast Tumor single patient yes 10 yr.  6.1% 0.0%  0.2% 1.7%  8.0% 16 Breast Tumor single patient yes 10 yr.  4.7% 0.1%  0.9% 2.0%  7.8% 17 Breast Tumor single patient yes 10 yr.  2.6% 0.1%  0.4% 0.9%  3.9% 18 Breast Tumor single patient yes 10 yr.  1.6% 0.1%  0.9% 1.0%  3.6% 19 Breast Tumor single patient yes 10 yr.  3.2% 0.2%  1.5% 1.6%  6.5% 20 Breast Tumor single patient yes 10 yr.  2.8% 1.5%  7.2% 1.3% 12.8% 21 Breast Tumor single patient yes 10 yr.  2.3% 0.2%  1.3% 1.7%  5.5% 22 Breast Normal single patient yes 10 yr.  2.0% 0.2%  0.7% 0.6%  3.5% 23 Breast Normal single patient yes 10 yr.  1.0% 0.3%  2.6% 0.7%  4.6% 24 Breast Normal single patient yes 10 yr.  1.0% 0.1%  1.3% 1.0%  3.4% 25 Breast Normal single patient yes 10 yr.  1.0% 2.7% 17.7% 1.3% 22.6% 26 Breast Normal single patient yes 10 yr.  0.9% 2.7% 10.1% 0.4% 14.2% 27 Breast Normal single patient yes 10 yr.  0.4% 2.0% 10.3% 0.5% 13.2% 28 Breast Normal single patient yes 10 yr.  0.7% 0.7%  6.4% 1.0%  8.7% 29 Breast Normal single patient yes 10 yr.  0.5% 4.5% 18.2% 0.5% 23.7% 30 Breast Tumor single patient yes 10 yr.  2.9% 0.0%  0.2% 1.2%  4.3% 31 Breast Tumor single patient yes 10 yr.  2.0% 0.2%  1.2% 1.2%  4.6% 32 Breast Tumor single patient yes 10 yr.  2.4% 0.0%  0.4% 1.4%  4.2% 33 Breast Tumor single patient yes 10 yr.  6.7% 0.1%  0.5% 3.0% 10.4% 34 Breast Tumor single patient yes 10 yr.  1.7% 0.1%  0.8% 0.9%  3.6% 35 Breast Tumor single patient yes 10 yr.  1.0% 0.1%  0.7% 0.6%  2.4% 36 Breast Tumor single patient yes 10 yr.  2.7% 0.2%  1.5% 1.3%  5.6% 37 Breast Tumor single patient yes 10 yr.  1.7% 0.0%  0.4% 1.2%  3.3% 3 Breast tumor pool yes 1 month  0.4% 0.9%  4.9% 4.1% 10.4% 3 Breast tumor pool yes 1 month  1.7% 0.5%  2.3% 2.7%  7.1% 3 Breast tumor pool yes 1 month  0.6% 0.0%  0.0% 5.8%  6.4% 3 Breast tumor pool yes 1 month  0.8% 0.0%  0.0% 2.8%  3.6% 38 Breast Tumor single patient yes 10 yr.  0.1% 0.0%  0.1% 2.9%  3.1% 39 Breast Tumor single patient yes 10 yr.  0.0% 0.0%  0.0% 0.9%  0.9%

Table 17 summarizes the average, median, standard deviation (“s.d.”), minimum and maximum proportions observed for the fifty-five libraries listed in Table 16. The maximum total rRNA proportion of 37.4% is due to a high abundance of 5.8S rRNA, a sequence that had not been depleted using the methods of the instant invention. The maximum observed rRNA proportion for 28S rRNA is in the range of 15-18%. These higher than average proportions to date have been confined to a subset of normal patients and may reflect a higher starting abundance of 28S rRNA in these patients.

TABLE 17 Other Total Statistic 5.8S% 18S% 28S% rRNA % rRNA % Avg   4.4%   0.8%   3.2% 1.5%  10.0% Median   2.1%   0.1%   0.9% 1.3%   7.1% s.d.   7.1%   1.3%   4.8% 1.0%   8.1% Max  35.9%   5.0%  18.2% 5.8%  37.4% Min 0.046% 0.002% 0.003% 0.4% 0.941%

All references cited throughout the disclosure, including the examples, are hereby expressly incorporated by reference for their entire disclosure.

While the present invention has been described with reference to what is considered to be specific embodiments, it is to be understood that the invention is not so limited. To the contrary, the invention is intended to cover various modifications and equivalents included within the spirit and scope of the appended claims. 

1. A method of depleting RNA from a fixed, paraffin-embedded (FPET) sample, comprising: a) contacting a nucleic acid sample with a multiplicity of DNA probes comprising a plurality of DNA oligonucleotides complementary to a target RNA, wherein the plurality of DNA oligonucleotides specifically hybridizes to at least 80% of the entire full length sequence or to the entire full length sequence of the target RNA and forms a DNA-RNA hybrid; and b) contacting the nucleic acid sample with a ribonuclease (RNase) that specifically recognizes a DNA-RNA hybrid; wherein the RNase degrades the target RNA in the DNA-RNA hybrid.
 2. The method of claim 1, wherein the RNase is RNase H.
 3. The method of claim 1, wherein the target RNA is ribosomal RNA (rRNA).
 4. The method of claim 3, wherein the rRNA is 18S or 28S, or both.
 5. The method of claim 3, wherein the rRNA is 12S or 16S, or both.
 6. The method of claim 1, wherein the DNA oligonucleotides comprise from 60 to 80 bases.
 7. (canceled)
 8. (canceled)
 9. The method of claim 4, wherein the plurality of DNA oligonucleotides specifically hybridizes to the entire full length sequence of 18S rRNA, 28S rRNA, or both.
 10. The method of claim 5, wherein the plurality of DNA oligonucleotides specifically hybridizes to the entire full length sequence of 12S rRNA, 16S rRNA, or both.
 11. The method of claim 2, wherein the RNAse H is a thermostable RNAse H.
 12. The method of claim 1, wherein the nucleic acid sample comprises total RNA.
 13. The method of claim 1, wherein the nucleic acid sample comprises amplified RNA.
 14. The method of claim 1, further comprising the step of contacting the nucleic acid sample with a DNase, wherein the DNase degrades the DNA probe.
 15. The method of claim 14, wherein the DNase is DNase I.
 16. The method of claim 1, wherein the target RNA comprises less than 10% of all RNA present in the nucleic acid sample after degradation with RNase. 17.-33. (canceled)
 34. The method of claim 1, wherein the FPET sample is from a tumor.
 35. The method of claim 34, wherein the tumor is a cancer.
 36. The method of claim 35, wherein the cancer is selected from breast cancer, colon cancer, lung cancer, prostate cancer, hepatocellular cancer, gastric cancer, pancreatic cancer, cervical cancer, ovarian cancer, liver cancer, bladder cancer, cancer of the urinary tract, thyroid cancer, renal cancer, carcinoma, melanoma, and brain cancer.
 37. A method of preparing cDNA comprising: a) obtaining a nucleic acid sample; b) contacting the nucleic acid sample with a multiplicity of DNA probes comprising a plurality of DNA oligonucleotides complementary to a target RNA, wherein the plurality of DNA oligonucleotides specifically hybridizes to at least 80% of the entire full length sequence or to the entire full length sequence of the target RNA and forms a DNA-RNA hybrid; c) contacting the nucleic acid sample with a ribonuclease (RNase) that specifically recognizes a DNA-RNA hybrid; wherein the RNase degrades the target RNA in the DNA-RNA hybrid; and d) preparing cDNA from the nucleic acid sample. 38.-66. (canceled)
 67. The method of claim 1, wherein the plurality of DNA oligonucleotides comprises at least two oligonucleotides hybridizing to target RNA sequences that overlap one another.
 68. A composition comprising: (a) a mixture comprising a plurality of RNA molecules, comprising target and non-target RNA molecules; (b) a multiplicity of DNA probes comprising a plurality of DNA oligonucleotides capable of specifically hybridizing to the target RNA molecules in the mixture to produce DNA-RNA hybrids, wherein the plurality of DNA oligonucleotides is capable of selectively hybridizing along at least 80% of the entire full length sequence of the target RNA molecules; and (c) an RNase for cleaving the DNA-RNA hybrids, wherein the RNase is capable of degrading the target RNA molecules in the DNA-RNA hybrids, wherein, in the composition, the plurality of DNA oligonucleotides of (b) is selectively hybridized along at least 80% of the entire full length sequence of the target RNA molecules of (a). 